1. Field of the Invention
This invention relates to a picture decoding method and apparatus for decoding compressed picture data of a first resolution obtained on predictive coding by motion prediction in terms of a pre-set pixel block (macro-block) as a unit and on orthogonal transform in terms of a pre-set pixel block (orthogonal transform block) as a unit. More particularly, it relates to a picture decoding method and apparatus for decoding compressed picture data of the first resolution and for decimating the data to moving picture data of a second resolution lower than the first resolution.
2. Description of the Related Art
There is now going on the standardization of digital television signals employing the picture compression system, such as Moving Picture Experts Group phase-2 (MPEG2). Among the standards for digital television broadcast, there are a standard for standard resolution pictures, such as those with the number of effective lines in the vertical direction of 576, and a standard for high-resolution pictures, such as those with the number of effective lines in the vertical direction of 1152. Recently, there is raised a demand for a picture decoding apparatus for decoding compressed picture data of a high-resolution picture and for reducing the resolution of the compressed picture data by 1/2 to generate picture data of the picture data of standard resolution to display the picture data on a television monitor adapted to cope with the standard resolution.
There is proposed in a publication entitled “Scalable Decoder free of low-range Drift” (written by Iwahashi, Kanbayashi and Takaya, Shingaku-Gihou CS 94-186, DSP 94-108, 1995-01) a downdecoder for decoding a bitstream of, for example, MPEG2, obtained on predictive coding with motion prediction of a high-resolution picture and compression coding by discrete cosine transform, and for downsampling the picture to a picture of standard resolution. The conventional picture decoding apparatus, disclosed in this publication, is now explained with reference to FIGS. 37 to 39.
Referring to FIG. 37, this conventional picture decoding apparatus 100 includes an inverse discrete cosine transform unit 101, for processing a bitstream of a high resolution picture with 8 (number of coefficients as counted from the DC component in the horizontal direction)×8 (number of coefficients as counted from the DC component in the vertical direction), an adder 102 for adding a discrete cosine transformed high resolution picture and a motion-compensated reference picture, and a frame memory 103 for transient storage of the reference picture. The picture decoding apparatus 100 also includes a motion compensation unit 104 for motion-compensating the reference picture stored in the frame memory 103 with 1/2 pixel precision, and a downsampling unit 105 for converting the reference picture stored in the frame memory 103 to a picture of standard resolution.
This conventional picture decoding apparatus 100 reduces an output picture, obtained on decoding as a high resolution picture by inverse discrete cosine transform, by the downsampling unit 105, to output resulting picture data with the standard resolution.
Referring to FIG. 38, another conventional picture decoding apparatus 110 includes an inverse discrete cosine transform unit 111 for performing 8×8 inverse discrete cosine transform, as it substitutes 0 for the high-frequency components of the discrete cosine transform (DCT) block of the high resolution picture, an adder 112 for summing the discrete cosine transformed high resolution picture to the motion-compensated reference picture, and a frame memory 113 for transient storage of the reference picture. The conventional picture decoding apparatus 110 also includes a motion compensation unit 114 for motion-compensating the reference picture stored in the frame memory 113 with 1/2 pixel precision, and a downsampling unit 115 for converting the reference picture stored in the frame memory 113 to a picture of standard resolution.
This conventional picture decoding apparatus 110 performs inverse discrete cosine transform to obtain a decoded output picture, as a high-resolution picture, as it substitutes 0 for coefficients of high-frequency components among the totality of coefficients of the DCT block, and reduces the output picture in size by the downsampling unit 115 to output picture data of standard resolution.
Referring to FIG. 39, a further picture decoding apparatus 120 includes a decimating inverse discrete cosine transform unit 121 for executing e.g., 4×4 inverse discrete cosine transform, using only the coefficients of the low-frequency components of the DCT block of the bitstream of the high resolution picture, for decoding to a standard resolution picture, and an adder 122 for summing the standard resolution picture processed with decimating inverse discrete cosine transform and the motion-compensated reference picture. The third downdecoder also includes a frame memory 123 for transiently storing the reference picture and a motion compensation unit 124 for motion-compensating the reference picture stored by the frame memory 1023 with a 1/4pixel precision.
In this conventional picture decoding apparatus 120, IDCT is executed using only low-frequency components of all coefficients of the DCT block to decode a picture of low resolution from a picture of high resolution.
The above-described conventional picture decoding apparatus 100 performs inverse discrete cosine transform on the totality of the coefficients in the DCT block to obtain a high-resolution picture on decoding. Thus, the inverse discrete cosine transform unit 1001 of high processing capability and the frame memory 1003 of high storage capacity are needed. The second conventional picture decoding apparatus 110 performs discrete cosine transform on the coefficients in the DCT block to obtain a high-resolution picture on decoding, as it sets the high-frequency components of the coefficients to zero, so that a lower processing capacity of the inverse discrete cosine transform unit 111 suffices. However, the frame memory 113 of high storage capacity is yet needed. In contradistinction from these first and second downdecoders, the conventional picture decoding apparatus 120 performs inverse discrete cosine transform on the totality of the coefficients in the DCT block, using only coefficients of the low-frequency components of the coefficients in the DCT block, so that a low processing capability of an inverse discrete cosine transform unit 121 suffices. Moreover, since the reference picture of the standard resolution picture is decoded, a lower capacity of the frame memory 123 suffices.
Meanwhile, the display system of a moving picture in television broadcast is classified into a sequential scanning system and an interlaced scanning system. The sequential scanning system sequentially displays a picture obtained on sampling the totality of pictures in a given frame at the same timing. The interlaced scanning system alternately displays pictures obtained on sampling pixels in a given frame at different timings from one horizontal line to another.
In this interlaced scanning system, one of the pictures obtained on sampling pixels in a frame at different timings from one horizontal line to another is termed a top field or a first field, with the other picture being termed a bottom field or a second field. The picture containing the leading line in the horizontal direction of a frame becomes the top field, while the picture containing the second line in the horizontal direction of the same frame becomes the bottom field. Thus, in the interlaced scanning system, a sole frame is made up of two fields.
With the MPEG2, not only a frame but also a field can be allocated to a picture as a picture compressing unit in order to compress the moving picture signals efficiently in the interlaced scanning system.
If, in the MPEG2, a field is allocated to a picture, the resulting bitstream structure is termed a field structure, whereas, if a frame is allocated to a picture, the resulting bitstream structure is termed a frame structure. In the field structure, a DCT block is constituted by pixels in the field and discrete cosine transform is applied on the field basis. The processing mode of performing field-based discrete cosine transform is termed the field DCT mode. In the frame structure, a DCT block is constituted by pixels in the frame and discrete cosine transform is applied on the frame basis. The processing mode of performing field-based discrete cosine transform is termed the frame DCT mode. In the field structure, a macro-block is constituted from pixels in a field and motion prediction is performed on the field basis. The processing mode of performing motion prediction on the field basis is termed the field motion prediction mode. In the frame structure, a macro-block is constituted from pixels in a frame and motion prediction is performed on the frame basis. The processing mode of performing motion prediction on the frame basis is termed the frame motion prediction mode.
Meanwhile, in the conventional picture decoding apparatus 120, IDCT is applied to the input bitstream of the high resolution picture, using only low frequency components in the horizontal and vertical directions, for decoding the input bitstream of the high resolution picture and downsampling to a standard resolution picture. However, if the input bitstream is of the interlaced scanning system, the tendency is that severe picture quality deterioration is produced in edges and fine lines having high frequency components in the vertical direction. In particular, if a reference picture is used in e.g., MPEG, this picture quality deterioration is propagated to produce significant deterioration as drift noise. As a specified example, FIG. 40 shows an output picture, obtained as a B-picture on decoding a bitstream corresponding to an HD test sequence “opening ceremony” (1920×1088 pixels, interlaced scanning at 30 Hz), compressed to 18 Mbps, by a conventional picture decoding apparatus.